Among the simplest and probably most widely used antennas is the horn, with applications including use as a feed element for dish antennas, reflectors and lenses, as elements of phased array antennas, for calibration and gain measurements of other antennas and devices, and for electromagnetic compatibility (EMC) testing. The widespread applicability of horns arises from its relative simplicity, ease of construction, ease of excitation, versatility, large gain and performance.
Horn antennas are essentially flared waveguides that produce a uniform phase front larger than the waveguide itself. A commercially available horn antenna is the Model 3115, manufactured by ETS Lindgren. See http://www.ets-lindgren.com/. A three dimensional view of this antenna is shown in FIG. 1. FIG. 2 shows a bottom view, side view and rear view of the Model 3115. This antenna comprises a connection assembly 1000, an upper plate 1100, a lower plate 1200, and side plates 1001 and 1002. The dimensions shown are nominal dimensions for the ridged horn antenna designed for operation in the 1 to 18 giga-Hertz (gHz) frequency band. Thus, upper and lower plates 1100, 1200 are nominally 9.63 inches wide at the wide end 1210 of the flare and 3.63 inches at the narrow end 1220 (bottom view). Upper plate 1100 and lower plate 1200 are each at an angle of +/−13 degrees, 14 minutes from the horizontal, extending 6 inches from connection assembly 1000. This is referred to herein as a pyramidal horn since the horn formed by the plates is flared in both the E-plane and the H-plane. Connection assembly 1000 provides a connection 1050 to couple power to the antenna from a coaxial cable (not shown). A threaded stud 1003 is provided for mounting the antenna.
FIG. 1 also shows a ridge 1250 attached to lower plate 1200. A second ridge 1150 of identical contour is attached to upper plate 1100. A side view and an edge view of a ridge 1150 or 1250 are shown in FIG. 3. The ridge exhibits a nominal edge thickness of 0.3550 inches and a nominal length of 7.5 inches. The ridge also exhibits a curvature or flare with nominal coordinates in inches as follows:
X0.0000.50001.0001.5002.0002.5003.0003.500Y0.0000.0000.0160.0320.0490.0850.1330.200X4.0004.5005.0005.5006.0006.5007.000Y0.2900.4220.6050.8751.2651.8552.695At its widest point, the ridge is 1.66 inches wide. Further, the ridge termination 1151 coincides with the end 1210 of a plate 5100, 5200.
The implementation of ridges 1150 and 1250 vastly extends the usable bandwidth of the basic horn antenna. Adding ridges to the horn antenna increases its bandwidth by lowering the cut off frequency of the dominant mode, while raising the cut off frequency of the next higher order mode. A gain pattern for the Model 3115 antenna is shown in FIG. 4, which shows a substantial gain over the frequency range between one and eighteen gHz. The Voltage Standing Wave Ratio (VSWR) for this frequency range is shown in FIG. 5, and the half power beam width is shown in FIG. 6.
A typical normalized radiation pattern of the ridge horn antenna is shown in FIGS. 7, 8 and 9, corresponding to 3, 12, and 17 gHz respectively. The preferred pattern is one in which the maximum power is delivered on the main axis (zero degrees), with monotonically decreasing power over a wide angular sector off the main axis. As shown in FIGS. 7, 8, and 9, as frequency increases, the main lobe of the antenna pattern becomes narrower and side lobes increase in power. Moreover, as reported in a recent technical journal, when the frequency of operation increases, the amplitude of off-axis side lobes increases and eventually surpasses the on-axis power. See IEEE Transactions on Electromagnetic Compatibility, Vol. 45, No. 1, February 2003, pages 55–60, Bruns, et.al.
Thus, although the standard ridged horn antenna provides usably high gain over a very broad frequency range, its directivity deteriorates at the high frequency end of that range. This is undesirable in most applications especially when the ridged horn antenna is used for calibration, gain measurements, or EMC testing. For EMC Immunity or susceptibility measurements it is also desirable to have the main lobe of the pattern wide enough to illuminate the equipment being tested, the narrow beam of the 3115 antenna is not well suited for this purpose. Improvement of an antenna's directivity without an increase in the VSWR within the frequency range of operation is difficult. Thus, what is needed is a ridged horn antenna that exhibits improved directivity at the high end of the frequency range for which its gain remains usably high, while providing a relatively low VSWR across the frequency range of operation.